Transmitter-receivers for at least two different frequency ranges are used, for example, for cordless telephones or in the mobile radio domain. The various frequency ranges then relate to, for example, the transmission frequency band and reception frequency band. FIG. 1 shows a circuit diagram of a transmitter-receiver in a mobile radiotelephone. An antenna 1 is used then for transmitting and receiving data. To avoid a transmit signal ending up in the receiver channel 2, the transmitter channel 3 and the receiver channel 2 are to be separated. For this purpose, a duplex filter 4 may be used which comprises two sharp-edged bandpass filters 5 and 6. A bandpass filter 5 is then tuned to the reception frequency band and the other filter 6 to the transmission frequency band. As a result, a transmit signal does reach antenna 1, because the filter 6 has little attenuation for the transmission frequency, but does not reach the receiver channel 2, because the filter 5 blocks this frequency band. A received signal from the antenna 1 is only slightly attenuated by the bandpass filter 5 and thus reaches the receiver channel 2, whereas it is strongly attenuated by the filter 6. Consequently, the duplex filter 4 is to have very small transit losses and a very sharp-edged filter behavior. Furthermore, an active electronic switch 7 (for example, realized by means of PIN diodes) may be used for separating the transmitter channel 3 and receiver channel 2 on the antenna 1, as is represented in FIG. 2. This type of changeover needs an additional control signal for the switch 7, which is to be generated in a costly manner. Furthermore, very strict requirements are made on the switches because very fast switching is to be realized. Furthermore, filters are to be provided which filter the respective frequency band for the transmitter channel 3 and receiver channel 2.
European patent application EP 0 481 986 has disclosed a communication radiotelephone which contains the transmitter and the receiver in one and the same housing, having a first antenna connected to the transmitter and tuned to the transmitter, and a second antenna connected to the receiver and tuned to the receiver, while the two antennas are separated from each other. The communication radiotelephone comprises a housing accommodating a transmitter and a receiver. The communication radiotelephone can transmit and receive simultaneously. A first antenna is connected to the receiver and a second antenna to the transmitter, while the transmitter and the receiver operate in different frequencies. The antennas tuned to the different frequencies are each connected to the receiver and transmitter, respectively via a filter. A separation of transmitter and receiver channel is then achieved in that the transmitting and receiving antennas have asymmetrical radiation characteristics and the position of the radiation characteristic is selected such that the antenna effect is always turned away from the transmitter and receiver. This, however, is highly disadvantageous in devices for transmission systems transmitting and receiving in the same direction. Furthermore, in most cases corresponding radiation characteristics for transmission and reception in a certain direction are required and/or desired.